tinuous operation to maximize the product per energy unit input and avoid the long reheat time after a shutdown. A new technique such as microwave energy might help in energy consumption as well as lower temperature operation.
Microwave Processing of Materials
Microwave reclamation, unlike thermal, is a volumet- ric technique, as the energy is capable of penetrating various materi- als (Fig. 1). Termal methods work by conduction of energy from the surface of a material to its core. In the case of large amounts of a material, from the surface of the bed depth to its core of the bed depth. Materials such as silica sand, alumina and other ceramics are not capable of absorbing microwave energy at low temperatures because these materials are electrically insulating and not magnetic, therefore they have no means to interact with the energy, making them transpar- ent to the energy.
Metals and other conducting
materials such as graphite have high electron densities that do not allow the electromagnetic field to penetrate. However, there can be “skin depth” penetration which results in eddy current loss that can allow the surface of the conductor to approach very high temperatures. But the bulk of the energy is reflected. Dipolar materials, magnetic
materials and certain semi-conducting materials can absorb microwave energy and dissipate that absorbed energy as heat. Tis is true of individual, monolithic and phase- pure materials as well as mixtures of materials and composites. Tis results in an absorbing
material being heated more thoroughly.
High Temperature Processing In microwave processing of
materials the material itself provides the heat, as opposed to having the heat provided by some added fuel source such as natural gas or energy from electrical-resistance heating.
Fig. 2. The dielectric loss of materials as a function of temperature is depicted.
Fig. 1. A simple view of microwave interaction with various classes of materials is shown.
If the material being processed is
capable of absorbing it, microwave targets that material, so the system’s containment structure and insulation mostly needs to maintain the tem- perature near the material. One of the substantial problems
in the advancement of the microwave processing is the lack of understand- ing of the materials system being processed.
Unlike water and other lower
temperature microwave-absorbing materials, most ceramics and other high temperature materials lack mechanisms for absorption of micro- wave at low temperatures. Hence the processing of these materials presents an enormous challenge for nearly all commercially available microwave equipment, and particularly in con- tinuous processing where a material can be exposed to different tempera- tures as that material travels through the kiln. Processing of ceramics and high temperature materials typically must
be pre-heated until these materials become “dielectrically lossy” or absorbing of micro- wave energy. Figure 2 represents the dielectric loss or microwave absorption of high tempera- ture materials as temperature increases.
Reclamation of Sand In addition to insulating
ceramics such as silica and alumina, cross-linked polymer resins like cured phenolic resins also exhibit similar behavior. Typi- cally these materials begin to absorb microwave energy around 482-572F (250-300C) whereas silica sand begins to absorb microwave energy around 1,382-1,472F (750-800C). Tis presents an opportunity to
process waste metalcasting sand with- out significantly increasing the sand temperature by targeting the materials that absorb microwave energy as the sand remains relatively transparent. Materials such as phenolic resins and the surface water, as well as bound or gallery water, of bentonite clay can be processed at lower temperatures than gas-fired furnaces.
The Microwave Concept Te developmental rotary kiln
concept allows for simultaneous application of thermal heating and microwave energy in the processing of waste foundry sand. Te simultaneous application of
thermal and microwave energy, also called “hybrid” processing, has been investigated in several concepts. A combination rotary system devel- oped by Ontario Hydro Technolo- gies, Burlington, Ontario is shown in Figure 3, using a preheating rotary section followed by a micro- wave rotary system. Both are novel in design, but neither has found wide-ranging use because of energy consumption from combining microwave energy, natural gas and/or resistive heating. Te investigation at MMP
and Grede uses designed ceram- ics that absorb microwave energy at relatively low temperatures and are capable of achieving very high
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